The difficult part here is to know what changes have actually been done to the schema so you can undo them. There is no magic undo button, but here’s a working procedure to remove any unwanted changes.

Open the exported xml file in your favorite text editor, as this file will contain only the changes made at the site collection level

Look for the SearchSchemaConfigurationSettings node

In the ManagedProperties nodes you will find new managed properties which have been created, in the image below a new one named MikaelKnowsHisStuff

Re-crawl content to weed out values from the index

If you have made mappings to existing managed properties you will see these listed in the Mappings node and have to examine and possible compare with a blank site to see which have been added or removed.

Unfortunately it’s not a simple undo operation, but at least possible. This is also why I strongly advocate to re-use the reusable managed properties for your custom use, as it’s quite easy to see any mappings done in the UI, as long as you know which ones might be globally mapped.